The invention relates to apparatus for the production of directionally solidified castings and castings produced by means of such apparatus.
An inexpensive process in which mold shells having integrated heat sources are used (F. Staub et al, Technische Rundschau Sulzer 3/1988, p. 11) is known for the production of relatively small directionally solidified castings (length in the direction of solidification less than about 15 cm). The integrated heat sources are, for example, additional cavities in the mold shell which, filled with superheated melt, enable casting to be carried out without heating means (susceptors) in the casting chamber. The mold shell itself and the mold thermal insulation which must be used, also contribute to the integrated heat sources.
The mold shell has a bottom aperture which is closed by a flat cooling plate in the casting chamber. Solidification of the melt starts at this aperture during casting. The cooling plate acting as a heat sink and the integrated heat sources form the poles of a temperature field which allows a "uni-directional" heat flow and hence a directional solidification. In the known processes the cooling plate forms a horizontal plane with respect to which the dendrites forming on solidification have a substantially vertical growth direction.
In castings made from nickel based alloys, e.g. for turbine blades for aircraft jet engines, the elongation strengths in the direction of the dendrites and hence the lives of the components during operation are greatly improved as compared with polycrystalline castings. Since the dendrites will be substantially radially oriented in the turbine wheel blades, the wheel must be assembled from individual directionally solidified castings. The production of the wheel would be simplified if it were possible to cast components having directionally solidified zones whose texture structures have different orientations. It is the object of the invention to provide an apparatus which allows the production of such components.